Cytochrome P-450, one of the key components of the mixed-function oxidase system has been found in multiple forms following induction by specific inducers. Hybridomas were prepared by fusion of myeloma cells with spleen cells of mice immunized with 3-methylcholanthrene-treated rat cytochrome (MC-P-450). Thirty-seven clones among 66 independent hybrid clones obtained in a selective medium produced monoclonal antibodies (MAB) to MC-P-450. Five of the six hybrid clones tested produced IgGl in culture and the sixth clone produced IgM. The MAB found are specific for MC-P-450 with respect to protein binding, precipitation and inhibition of metabolic activity. The MABs interacted with Beta-napthoflavone induced cytochrome P-450, as well as with MC-P-450, but not with phenobarbital-induced cytochrome P-450. The MAB to MC-P-450 exhibited very similar inhibitory activity toward both the mixed-function oxidase and 7-ethoxy-coumarin deethylase of microsomes. The MABs will be very useful for identification of multiple forms of cytochrome P-450 and for investigation of individual differences in susceptibility to chemical carcinogens.